Archaeological Chemistry IV - American Chemical Society

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of Nile Sediments Ralph O. Allen , Hany Hamroush , and Michael A. Hoffman 1

1,3

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Department of Chemistry, University of Virginia, Charlottesville, VA 22901 Earth Sciences and Resource Institute, University of South Carolina, Columbia, S C 29208

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The ability to distinguish Nile sediments from different time periods and different sources allows identification of sediments at different archaeological sites. Nile river sediments representing a range of geological ages from Predynastic sites at Hierakonpolis were analyzed by instrumental neutron activation analysis. Some Nile sediments and the Egyptian pottery produced from them can be distinguished on the basis of the relative distributions of the lanthanides. Other groups of sediments are chemically similar but have differences due to "dilution" by sand (mostly quartz). Fractions of Nile sediments separated on the basis of size showed that most of the trace elements were in the fine-grained fraction, but the trace elements in the sand-size fraction seemed to reflect the relative contributions of the different Nile tributaries and the geology of the drainage basins.

THE USE OF FIRE TO TRANSFORM CLAY PASTE

into s o l i d c e r a m i c vessels m a y have b e e n one of the earliest efforts at c h e m i s t r y . Since the earliest t i m e s , pottery has b e e n made b y u s i n g clays f o r m e d b y the w e a t h e r i n g of rocks. S e d i m e n t a r y deposits c o n t a i n i n g clay m i n e r a l s also contain fragments of other m i n e r a l s that are b r o k e n from the source rocks as they weather. T h e c h e m i c a l c o m p o s i t i o n of the sediments u s e d as a clay source d e t e r m i n e d some of the characteristics of the p o t t e r y that was p r o d u c e d . 3

Present address: Geology Department, Faculty of Science, Cairo University, G i z a , Egypt 0065-2393/89/0220-0033$06.00/0 © 1989 A m e r i c a n C h e m i c a l S o c i e t y

In Archaeological Chemistry IV; Allen, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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W i t h h i g h - c a l c i u m (calcareous or marl) sediments f o u n d n o r t h of the D e a d Sea, potters p r o d u c e d h i g h l y v i t r i f i e d (glassy) wares (J). A t the same t i m e (ca. 3500 B . C . ) , potters along the N i l e R i v e r u s e d firing conditions s i m i l a r to those u s e d for h i g h - c a l c i u m sediments, b u t because the sediments generally c o n t a i n e d less t h a n 3 % C a O , the pottery was far less v i t r i f i e d t h a n that p r o d u c e d f r o m m a r l sediments. P o t t e r y f r o m these two locations differed i n color as w e l l as i n the degree of vitrification. T h e r e l a t i v e l y i r o n - r i c h N i l e sediments w e r e u s e d to p r o d u c e a range of r e d , buff, b r o w n , orange, a n d e v e n black pottery, i n contrast to the w h i t e , gray, a n d black wares p r o d u c e d from l o w - i r o n calcareous sediments. M i n e r a l o g i c a l c o m p o s i t i o n was also i m p o r t a n t . S e d i m e n t s c o n t a i n i n g too m u c h coarse-grained m a t e r i a l (e.g., q u a r t z or other minerals) m a d e p o o r pottery unless the coarser grains w e r e r e m o v e d to e n r i c h the clay content of the sediment. F i n e - g r a i n e d clay sediments n e e d e d coarser g r a i n e d sand or o t h e r t e m p e r a d d e d to a i d i n w o r k i n g the paste a n d firing the pottery. I n some cases, the m i n e r a l fragments that o c c u r r e d naturally w i t h the clay minerals s e r v e d as a natural t e m p e r , a n d as s u c h , may p r o v i d e a basis for d i s t i n g u i s h i n g the source o f the sediment. T h e identification of clay sources a n d the observation that a d d i t i o n a l components w e r e a d d e d to p r o d u c e a particular type of pottery helps enhance o u r u n d e r s t a n d i n g of the t e c h n o logical practices o f the ancient potters. I n a geochemical sense, sediments t e n d to represent an average of the geological terrain from w h i c h they are d e r i v e d . T h e clay minerals are f o r m e d b y the c h e m i c a l w e a t h e r i n g of r o c k - f o r m i n g minerals l i k e feldspars a n d ferromagnesian silicates. O t h e r m i n e r a l s , l i k e q u a r t z , r e m a i n u n a l t e r e d b u t are gradually b r o k e n i n t o small fragments that m i x w i t h the clays. T h u s , a r i v e r s e d i m e n t is a m i x t u r e of w e a t h e r e d minerals (clays) a n d r e l i c minerals d e r i v e d from the surface rocks i n the drainage b a s i n . T h e c h e m i c a l c o m p o s i t i o n o f a r i v e r s e d i m e n t is a reflection of the average c h e m i c a l c o m p o s i t i o n of rocks i n the drainage b a s i n . S e d i m e n t s f r o m different rivers m i g h t differ i n c h e m i c a l a n d m i n e r a l o g i c a l c o m p o s i t i o n , whereas sediments from a single r i v e r s h o u l d be fairly u n i f o r m i n c h e m i c a l c o m p o s i t i o n i f s i m i l a r g r a i n - s i z e d fractions are c o m p a r e d . S e d i m e n t s d e p o s i t e d at different locations along the same r i v e r can differ i n m i n e r a l o g i c a l a n d c h e m i c a l c o m p o s i t i o n . T h e s e differences result f r o m the differential s e d i m e n t a t i o n or d e p o s i t i o n of i n d i v i d u a l m i n e r a l grains from the r u n n i n g w a t e r that transports the grains d o w n s t r e a m . T h e patterns of d e p o s i t i o n d e p e n d u p o n the grain size (which is r e l a t e d to the mineral's hardness), the d e n s i t y of the particles, a n d the water's flow rate. T h e N i l e R i v e r p r o v i d e s a good example of the similarities and differences b e t w e e n sediments from the same r i v e r . T h e c o m m o n o r i g i n of these N i l e sediments is c o n f i r m e d b y the h i g h degree of correlation i n the e l e m e n t a l compositions b e t w e e n different E g y p t i a n N i l e s e d i m e n t samples (2).


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These samples are similar i n c o m p o s i t i o n , b u t they have subtle c h e m i c a l differences. T o u n d e r s t a n d the sediments d e p o s i t e d along the N i l e , i t is h e l p f u l to realize that as the r i v e r flows t h r o u g h E g y p t , i t is flowing t h r o u g h a v e r y large delta. S e d i m e n t s f r o m two d i s t i n c t drainage basins are c a r r i e d b y the B l u e N i l e a n d the W h i t e N i l e u n t i l they m e r g e a n d f o r m the N i l e . Before the construction of the A s w a n H i g h D a m , m i l l i o n s of tons of s u s p e n d e d matter was c a r r i e d b y the N i l e as it e m p t i e d out onto this d e l t a a n d d e p o s i t e d as sediments along the banks of the r i v e r . I n the geological past, these sediments w e r e d e p o s i t e d o n top of the earlier s e d i m e n t a r y deposits that reflected earlier e n v i r o n m e n t s s u c h as smaller deltas, a l l u v i a l fans, flood plains, a n d e v e n ocean floors. T h e r e s u l t i n g c o m p l e x sequence of s e d i m e n tary deposits is often difficult to i n t e r p r e t . Part of the w o r k d e s c r i b e d i n this chapter was a i m e d at s t u d y i n g the N i l e sediments i n one locality to d e t e r m i n e w h e t h e r the c o m p l e x deposits c o u l d b e differentiated b y c h e m i c a l means. I d e n t i f y i n g the sources of sediments c h e m i c a l l y w o u l d b e valuable i n a n s w e r i n g a n u m b e r o f archaeological questions. F o r example, the s i m i l a r i t y i n the compositions of the m o d e r n N i l e sediments suggested that the c o m positions of early E g y p t i a n p o t t e r y made from N i l e clays w o u l d b e s i m i l a r a n d of little use for provenance studies (see ref. 3); therefore, i t was somewhat u n e x p e c t e d w h e n e a r l i e r (4-5) studies of P r e d y n a s t i c E g y p t i a n pottery s u g gested that there w e r e some geochemically significant differences b e t w e e n ancient N i l e sediments, d e p o s i t e d some 40,000 years ago, a n d those d e posited more recently. A s a part o f o n g o i n g studies at the i m p o r t a n t P r e d y n a s t i c E g y p t i a n sites at H i e r a k o n p o l i s , w e have a t t e m p t e d to u n d e r s t a n d h o w the sediments d e p o s i t e d i n this area have v a r i e d spatially a n d o v e r t i m e . T h e results of these geochemical studies, s u m m a r i z e d i n this chapter, indicate h o w a d e t a i l e d u n d e r s t a n d i n g of the sediments can enhance o u r k n o w l e d g e of the e n v i r o n m e n t a l e v o l u t i o n of the local landscape, h e l p to relate separate archaeological sites i n an area, a n d p r o v i d e a basis for comparisons of the pottery p r o d u c e d i n the area.

Archaeological Significance of Hierakonpolis Since the t i m e of the G r e e k historian H e r o d o t u s (484-425 B . C . ) , scholars have speculated o n the role of the N i l e i n the o r i g i n a n d d e v e l o p m e n t of E g y p t i a n c i v i l i z a t i o n . O n e of the most i m p o r t a n t sites at w h i c h this role can be investigated is H i e r a k o n p o l i s , w h e r e b o t h ancient legends a n d archaeological e v i d e n c e suggest the first leaders of u n i f i e d E g y p t i a n state e m e r g e d i n the fourth m i l l e n n i u m B . C . (6).


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T h e n a m e H i e r a k o n p o l i s ( G r e e k for C i t y of the H a w k ) was o r i g i n a l l y u s e d i n a m o r e r e s t r i c t e d sense to refer to the A r c h a i c - O l d K i n g d o m (ca. 3 1 0 0 - 2 2 3 0 B . C . ) w a l l e d t o w n o f " N e k h e n , " w h i c h is located i n the m o d e r n N i l e flood p l a i n along the west bank of the N i l e r i v e r about 650 k m south of C a i r o . I n this chapter, the n a m e H i e r a k o n p o l i s w i l l be u s e d to embrace a larger archaeological a n d geographical r e g i o n s u r r o u n d i n g the t o w n of N e k h e n , i n c l u d i n g archaeological sites r a n g i n g i n age from L o w e r P a l e o l i t h i c to G r e c o - R o m a n (ca. 2 5 0 , 0 0 0 - 3 1 B . C . ) . T h e b e s t - k n o w n a n d most i m p o r t a n t aspects of this area are the extensive P r e d y n a s t i c (ca. 4 0 0 0 - 3 1 0 0 B . C . ) settlement a n d c e m e t e r y complexes. L o n g before a n d d u r i n g the unification of the E g y p t i a n state (ca. 3100 B . C . ) , H i e r a k o n p o l i s was a t o w n of religious a n d p o l i t i c a l d i s t i n c t i o n a n d m a y have served as the P r e d y n a s t i c capital of U p p e r E g y p t . W h e r e a s little was k n o w n about the nature a n d d i s t r i b u t i o n of sites i n the N i l e flood p l a i n , the large areas o f P r e d y n a s t i c occupation (stretching for about 1.5 k m along the l o w desert o n the edge of the m o d e r n c u l t i v a t i o n zone) have b e e n the subject of considerable investigation (e.g., refs. 6-8). T h e P r e d y n a s t i c settlements a n d c e m e t e r y complexes also e x t e n d about 3.5 k m west of N e k h e n into the w e s t e r n desert along an ancient drainage course k n o w n as the G r e a t W a d i or W a d i A b u l Suffian. T h e " r o y a l " c e m e t e r y c o m p l e x (Tombs 1 a n d 2), w h e r e a considerable a m o u n t of fine P l u m R e d W a r e ( P R W ) was f o u n d , was located i n the G r e a t W a d i . T h e P l u m R e d W a r e p o t t e r y appeared to have b e e n fired at sites along the n o r t h e r n side of the G r e a t W a d i (localities 39 a n d 59) o n the u p p e r beds of ancient (Cretaceous) sediments (variegated shales a n d sandstone) that are a part of the N u b i a n formation. T h e N u b i a n s e d i m e n t a r y formation has a different o r i g i n than the N i l e sediments, a n d the two types of sediments can be easily d i s t i n g u i s h e d o n the basis of trace e l e m e n t contents (4). A s F i g u r e 1 shows, these ancient variegated shales a n d ferrugineous sandstone beds are exposed i n parts of the l o w desert surface west of the c u l t i v a t i o n zone, as w e l l as i n the h i g h desert areas that b o r d e r the G r e a t W a d i . I n most areas o f the w a d i floor a n d the l o w desert area, the N u b i a n formation is c o v e r e d b y a n average of 5 - 7 m of Pleistocene N i l e silts (9). T h e s e o l d e r N i l e sediments represent different episodes i n the e v o l u t i o n o f the N i l e R i v e r (10-13). T h e oldest N i l e sediments (called Protonile) i n the area, exposed at h i g h (about 125 m above sea level) Pleistocene terraces, w e r e deposited d u r i n g the L o w e r to M i d d l e P a l e o l i t h i c p e r i o d (JO). T h e m o r e recent (Neonile) sediments i n the area i n c l u d e the M a s m a s formation (clay a n d sandy silts d e p o s i t e d some 40,000 years ago). N e o n i l e sediments c a l l e d the Sahaba formation are y o u n g e r (ca. 20,000 B . C . ) , a n d , as seen i n F i g u r e 1, these sediments cover m u c h of the area b o r d e r i n g the m o d e r n flood p l a i n o n w h i c h most of the P r e d y n a s t i c sites are located. Some c h e m i c a l differences are seen b e t w e e n these two N e o n i l e s e d i m e n t a r y units



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Figure 1. Map of Hierakonpolis. Some of the important archaeological sites are shown along with the sedimentary formations exposed in the area. Tombs 1 and 2 mark the Predynastic "royal" cemetery. (4-5). A final N e o n i l e sediment is the y o u n g e r ozone that is exposed i n o n l y one area near anticipated that most of the E l - K a b formation the m o d e r n N i l e sediments o n the flood p l a i n u n d e r these c u l t i v a t e d soils.

(ca. 9000 B . C . ) E l - K a b l i t h locality 24 (14, 15). It was sediments w e r e c o v e r e d b y a n d w o u l d be found d i r e c t l y

Experimental Details Samples. T h e A r c h a i c - O l d K i n g d o m w a l l e d t o w n of N e k h e n was i n the m o d e r n flood p l a i n . T h i s region (known locally as K o m e l A h m r ) , first d u g b y the E n g l i s h m e n J . E . Q u i b e l l a n d F. W . G r e e n i n the late 1890s, was investigated b y an i n t e r d i s c i p l i n a r y team i n 1984. Several trenches w e r e excavated along a s m a l l canal b e t w e e n the site a n d the edge of the nearby desert. A m a n u a l auger was u s e d to sample the sediments i n the 16 locations shown i n F i g u r e 2. V i s u a l examination of the sediments a n d the artifacts (pottery) f o u n d at each l e v e l s h o w e d that the area a r o u n d N e k h e n was m o r e stratigraphically c o m p l e x than anticipated.



Figure 2. Map of Kom el Ahmr area at Hierakonpolis showing the partially excavated archaeological including the old temple and the numbered locations from which test cores were taken.


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B e l o w the sediments d i s r u p t e d b y c u l t i v a t i o n , a u n i t ( U n i t A) was f o u n d that was about 120 c m t h i c k a n d that c o n t a i n e d a d i s o r d e r l y m i x t u r e (almost an inverse t i m e sequence) of R o m a n , Pharaonic, and P r e d y n a s t i c sherds. T h e next 15 c m or so was a c l a y - r i c h N i l e silt ( U n i t B) i n w h i c h t h e r e was a n o r m a l c e r a m i c sequence w i t h sherds d a t i n g from about 300 B . C . to about 2500 B . C . Table I describes the t y p i c a l core sample. T h e layers of sediments w e r e c o m p a c t e d a n d , i n some cases, partially h a r d e n e d b y a calcareous c e m e n t , b u t i n a l l cases, the samples c o u l d b e easily c r u m b l e d . B e l o w U n i t B a silty sand layer ( U n i t C ) was e n c o u n t e r e d i n the cores taken b e t w e e n the m o u t h of the G r e a t W a d i a n d the center o f the K o m e l A h m r . U n i t C c o n t a i n e d a sequence of O l d K i n g d o m to late P r e d y n a s t i c artifacts d a t i n g f r o m about 2500 B . C . to 3200 B . C . B e l o w U n i t C , a v e r y compact, w e l l - s o r t e d t h i c k layer of N i l e clay a n d sand was e n c o u n t e r e d ( U n i t N ) . N o cores or trenches r e a c h e d b e l o w this sedimentary deposit. U n i t N c o n t a i n e d occasional P r e d y n a s t i c c e r a m i c a n d flint artifacts. Samples w e r e c o l l e c t e d from each 1 0 - 1 5 - e m auger cut w i t h i n each s e d i m e n t a r y u n i t i n each core or t r e n c h . I n a d d i t i o n , n u m e r o u s samples of the N e o n i l e deposits a n d other sediments f r o m the n e a r b y l o w desert a n d G r e a t W a d i w e r e taken for analysis. N i l o t i c sediments w e r e s a m p l e d across the r i v e r f r o m H i e r a k o n p o l i s from the N e k h e b formation at E l - K a b ( 7 8 . 5 - 8 0 m above sea level), w h i c h was thought to have b e e n d e p o s i t e d b e g i n n i n g about 11,000 years ago (15). I n a d d i t i o n , a series of 27 N i l o t i c sediments w e r e c o l l e c t e d f r o m along a 3 5 0 - k m stretch of the N i l e V a l l e y i n U p p e r E g y p t . Because the N i l e s e d i -

Table I. Definition of Sedimentary Units from a Typical Core Sample from Vicinity Between K o m el A h m r and the Edge of the Cultivation Zone Appropriate Thickness Cultural Approximate Approximate Unit (cm) Dates Period Sediment Overburden -100 Modern Cultivation Zone 1968-present A 120 Ca 220-320 B.C. Ptolmaic Anthropic Nile" B 15 to 2500 B.C. New Kingdom Nile Sharp contact C -100 2500-3100 B.C. Archaic Wadi Deposits (Dynasty 1-2) -50 3100-3200 B.C. Protodynastic Gradual contact N -75 3200-3400 B.C. Gerzean Nile -75 3400-3700 B.C. Amratian Nile at least 150 3700 - ? Badarian Nile b

NOTE: Units are in order of depth from surface (at 82.23 m above sea level) to the bottom of the core (about 5 m below the surface). This layer appears to be the result of leveling the site for agricultural purposes (ca. 320-220 B.C.) as described in ancient writings. On the basis of earlier conclusions (14) drawn from site at El-Kab across the Nile from Nekhen, the deposition of this series of Nile sediments, which is called the Nekheb lithozone, probably began about 9000 B.C. 6


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merits originate from different geological terrains i n A f r i c a , samples of fine silts w e r e o b t a i n e d f r o m the W h i t e N i l e , the B l u e N i l e , a n d the r e g i o n of K h a r t o u m w h e r e these major r i v e r s j o i n to f o r m the N i l e before it

flows

n o r t h t h r o u g h E g y p t (16). Analytical Methods. Bulk samples of the Nile sediments (200-500 mg) were analyzed with instrumental neutron activation analysis (INAA). The basic procedure has been described elsewhere (4, 17). Samples and appropriate geological reference standards were irradiated for 1 h in the University of Virginia research reactor at a flux of 1.2 X 10 neutrons cm s . The samples were counted with a relatively thin Ge(Li) detector that provided high efficiency and resolution for low energies (0.86 keV full width at half maximum ( F W H M ) for a 122-keV Co-57 peak), but was sufficiently efficient for counting high-energy 7-rays (1.1% efficiency for 1.332M e V Co-60 7-ray relative to 3- X 3-in. N a l crystal). A l l samples were counted 4-7 days after irradiation and again 30-40 days after irradiation. As will be discussed later, the initial analysis suggested that the bulk samples were very similar in composition. Thus, to understand the differences that were observed in the Neonile sediments, a more detailed investigation of the different units from the region around K o m el Ahmr was begun. D r i e d bulk samples were examined under a binocular microscope to detect organic matter and to determine the grain size distribution. This examination was supplemented with scanning electron microscopy (SEM) to determine the lithology. Depending upon the relative proportions of sand and clay, representative samples of between 10 and 50 g were stirred with deionized distilled water for 10 min to remove any water-soluble salts (e.g., NaCl). After allowing the sediment to settle for 5-6 h, the water was decanted and the water wash cycle was repeated five more times. Following the final wash with water, the carbonate salts, which cemented some of the grains together, were removed by slowly adding a 5% HC1 solution until there was no effervescence. It was anticipated that this acid would also remove iron oxide coating from the mineral grains. After rinsing six times with distilled water, a sample ( about 1 g) was taken for analysis. The remaining material was warmed in a 10% H 0 solution and allowed to stand overnight to remove all organic matter. After washing with distilled water, the sediment was suspended in 425 m L of a solution that contained 2.35 g of sodium hexametaphosphate (Calgon) and stirred overnight. Within 30 s after stirring was stopped, the sample was filtered through a 63-jxm (4-phi mesh) screen to separate the sand-sized grains (0.063-2 mm) from the finer particles. Although this fraction is described i n this chapter as mud, it is a mixture of what sedimentologists would call the silt and clay fractions. After washing with distilled water five or six times, separate grain size fractions were dried (at 50 °C for 6 h) and sampled for analysis. A portion of the sand-sized fraction was mixed with 1,2-tetrabromoethane (specific gravity of 2.9). The less dense grains of quartz and feldspar floated on the surface and were separated from the more dense (heavy) minerals (mostly pyroxenes, amphiboles, opaque minerals, and epidotes). Both fractions were washed with acetone 20-25 times, washed with distilled water five or six times, and finally dried overnight at 80 °C. For each sediment, both fractions of the sand-sized material were then examined with binocular and polarizing light microscopes to determine the efficiency of the separations. The less dense fraction was white or clear and composed of fragments of quartz and feldspar. This fraction showed very little contamination of the grains by the darker mineral fragments that were found in the heavier fraction. The separation of specific minerals was not the goal of this study, so there was no effort to remove the small number of white quartz fragments from the dark heavy


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minerals. By making these separations, it was possible to analyze the relic minerals free from the clay minerals that carry most of the trace elements in these river sediments. During the analysis of these fractions, the concentration of B r was used to detect contamination of the grains. The results indicated that the extensive washing was sufficient to remove the tetrabromomethane.


Results and Discussion Nile Sediments. T h r o u g h o u t the m i l l i o n s of years of its h i s t o r y , the N i l e has c a r r i e d sediments to a n d t h r o u g h E g y p t . O n its nearly 7000-km course from c e n t r a l a n d eastern A f r i c a to the E g y p t i a n D e l t a , s u s p e n d e d matter is c a r r i e d b y the N i l e R i v e r . Table II contains a s u m m a r y of data that suggests that the compositions of the sediments that are d e p o s i t e d b y the N i l e along its banks are v e r y similar. W h e n the 27 samples of m o d e r n s e d i m e n t from along a 3 5 0 - k m stretch of the N i l e i n U p p e r E g y p t w e r e analyzed b y I N A A , there w e r e differences of as m u c h as 3 0 % i n the c o n centrations o f the 17 elements m e a s u r e d (nine of the rare earth e l e m e n t s , T h , U , a n d the elements i n T a b l e II). T h e ratios of the various trace e l e m e n t s , h o w e v e r , w e r e v e r y similar for each sample, a fact suggesting that the major differences w e r e d u e to variable amounts of quartz sand i n the sediments. Q u a r t z t y p i c a l l y yields l o w concentrations of the elements w h e n measu r e d b y this p r o c e d u r e , a n d therefore does not c o n t r i b u t e significantly to the o v e r a l l trace e l e m e n t c o m p o s i t i o n of the b u l k s e d i m e n t samples. T h u s , the presence of quartz i n sediments acts as a d i l u t a n t to t h e i r o v e r a l l trace e l e m e n t content. U n f o r t u n a t e l y , S i cannot b e m e a s u r e d b y this I N A A p r o Table II. Average Concentrations (by Weight) of Some Elements i n Bulk Sediment Samples from Hierakonpolis Sediment Formation*

Fe 0 (%) 2

Nile River Sediment Composite (27) S.D. E l Kab (3) Sahaba (5) Masmas (5) Protonile (5) Wadi (4) Nubian Sandstone (4) b

c

3

8.5 ±1.6 10.8 9.3 9.3 5.0 5.0 0.6

Na 0 (%)

Co (ppm)

Sc (ppm)

Cr (ppm)

Hf (ppm)

1.5 ±0.2 2.0 2.8 1.6 1.0 1.0

28 ±6 33 29 32 13 15

19 ±2 21 21 18 7 10

132 ±17 122 244 148 62 78

6 ±1 8 6 6 6 5.5

36 ±4 43 33 33 23 23

6

1

n.a.

3.2

6

2

0.05

La (ppm)

NOTE: All samples were measured by INAA. The elemental concentrations of Fe and Na were measured but have been calculated as the weight percent oxide, although this calculation probably does not represent the exact chemical nature of these species. "The number of separate samples of each type shown in parentheses. ^The average for 27 modern sediment samples from along a 350-km length of the Nile in Upper Egypt. The standard deviation for the NRSC samples is typical of that observed for all the other groups. Mixture or materials (Old Nile and Nubian Formation) washed from the Great Wadi. c


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c e d u r e . A n i n v e r s e c o r r e l a t i o n b e t w e e n S i a n d the trace elements w o u l d prove that d i l u t i o n b y sand was the source of the o b s e r v e d variations i n these N i l e sediments.


Averages for the 27 b u l k samples are g i v e n i n Table I I for some of the elements m e a s u r e d . T h e s e averages, c a l l e d N i l e R i v e r S e d i m e n t C o m p o s i t e ( N R S C ) , are v e r y s i m i l a r to the averages for m o d e r n N i l e sediments c o m p i l e d b y T o b i a a n d Sayer (2). T h i s s i m i l a r i t y c l e a r l y confirms that, other t h a n the variations due to grain size (sand content), the m o d e r n N i l e silts are q u i t e uniform i n composition. R e c o g n i t i o n of the mechanisms b y w h i c h trace elements are p a r t i t i o n e d into m i n e r a l s suggests the i m p o r t a n c e of l o o k i n g at the relative d i s t r i b u t i o n s of groups of elements that have similar c h e m i c a l behavior. T h e rare earth elements ( R E E ) , or lanthanides, have b e e n particularly useful because t h e y usually o c c u r as t r i v a l e n t cations that differ from each other o n l y i n i o n i c size. E a c h m i n e r a l , as i t is f o r m e d , partitions the R E E a n d other trace elements into its crystal lattice o n the basis of i o n i c size a n d charge. T h e R E E are d i s t r i b u t e d i n m i n e r a l s o n the basis o f size, a n d the total c o n c e n tration i n a rock d e p e n d s u p o n the m i n e r a l s that are present. I n some cases, there is an " a n o m a l y " i n the b e h a v i o r of E u , w h i c h can b e separated from the others w h e n it is r e d u c e d partially to the 2 + oxidation state. T h e relative d i s t r i b u t i o n s of R E E i n geological materials are often r e p r e s e n t e d b y p l o t t i n g the n o r m a l i z e d * R E E concentration (concentration of e l e m e n t i n the rock d i v i d e d b y the average concentration of that e l e m e n t i n c h o n d r i t i c meteorites) as a f u n c t i o n of atomic n u m b e r ( w h i c h is i n v e r s e l y p r o p o r t i o n a l to the radius of the 3 + ion) as shown i n F i g u r e 3. I n F i g u r e 3 the R E E concentrations i n the N i l e sediments are all s i m i l a r , so the R E E patterns A , B , a n d C are offset from each other. T h e s e are s e m i l o g plots. P a t t e r n D is the N R S C , an average o f 27 m o d e r n N i l e s e d i ments f r o m U p p e r E g y p t . P a t t e r n A is the average of five samples from the M a s m a s formation (the oldest of the N e o n i l e sediments). P a t t e r n C is the average o f five samples from the Sahaba formation, a n d P a t t e r n B is the average o f t h r e e samples f r o m the E l K a b formation a l l f r o m H i e r a k o n p o l i s . P a t t e r n E is the average of five samples o f the m u c h o l d e r P r o t o n i l e s e d i m e n t s , a n d P a t t e r n F is the average of four samples of the e v e n o l d e r (nonN i l e ) N u b i a n f o r m a t i o n sandstone. T h e t y p i c a l m o d e r n N i l e s e d i m e n t (shown as the N R S C ) shows an e n r i c h m e n t of L a relative to L u a n d the negative E u anomaly (concentration relative to S m a n d G d ) that is t y p i c a l of the earth's crust. T h e results for the average N i l e s e d i m e n t of U p p e r E g y p t ( N R S C ) c o m p a r e q u i t e w e l l w i t h the averages for the o l d e r N i l e sediments ( E l - K a b a n d Sahaba) f o u n d at H i e r a k o n p o l i s (Table II), a result suggesting that there * This normalization process helps highlight the effects of geochemical processes on the separation of this group of very similar elements (12, 19).


Nile Sediments


A L L E N ET AL.

r- l IB CE

I

r NO

I

i

I

I

I

5M EU GO TB

I

I

I

i

i

j

TB LU

Figure 3. Average rare earth element (REE) distributions in sediments from Hierakonpolis.



44


has b e e n little change over the last 20,000 years. H o w e v e r , i f the relative concentrations are e x a m i n e d , there appear to be some differences i n the R E E d i s t r i b u t i o n patterns ( F i g u r e 3) for the o l d e r M a s m a s formation. A l t h o u g h the concentrations of L a , L u , a n d E u are v e r y s i m i l a r i n the M a s m a s s e d i m e n t , the E u is not anomalous b u t has a concentration that w o u l d b e expected i n m i n e r a l s w h e r e it was mostly present i n the + 3 oxidation state. I n the o l d e r P r o t o n i l e sediments at H i e r a k o n p o l i s , the R E E patterns are also different a n d have a p r o n o u n c e d negative E u anomaly a n d a l o w e r L a / L u ratio than the M a s m a s formation ( F i g u r e 3). T h e differences b e t w e e n P r o t o n i l e a n d the y o u n g e r N i l e sediments are also clear from the other elements i n T a b l e I I , a n d correlation diagrams l i k e that i n F i g u r e 4 for C o a n d Sc i n the b u l k samples. T h e results f r o m the N u b i a n sandstone are also s h o w n i n Table I I a n d F i g u r e 3 for c o m p a r i s o n to the various N i l e sediments at H i e r a k o n p o l i s . T h e s e ancient N u b i a n formation sediments, w h i c h contain large amounts of quartz, are c l e a r l y d i s t i n g u i s h a b l e . M a t e r i a l w a s h e d from the G r e a t W a d i is a m i x t u r e c o n t a i n i n g N u b i a n formation sediments (mainly gravel-sized) a n d the o l d e r N i l e sediments (mainly m u d a n d sand-sized) that h a d b e e n d e p o s i t e d o n the ancient w a d i floor. T h e s i m i l a r i t y b e t w e e n the average w a d i s e d i m e n t a n d the P r o t o n i l e sediments (Table II) suggests that the c o n t r i butions to the finer sand a n d m u d - s i z e d m a t e r i a l (older N i l e sediments) b y

10

15 20 Scandium

Figure 4. Concentrations of Co and Sc in bulk samples of sediments found at Hierakonpolis. The Nubian sandstone (•) and the Protonile sediments contain far less Co and Sc than the Neonile (Masmas, Sahaba, and El Kab) sediments ( + ). For comparison the average concentrations for Units B (•), N (x), C (*), and the NRSC (A) are shown. Although sediments from Units B and N are Neonile, the low concentrations suggest higher proportions of sand (dilutant) than in the older Neonile sediments from the low desert area.


3.

A L L E N ET AL.

45

Nile Sediments

the N u b i a n sandstone is small. T h e R E E d i s t r i b u t i o n patterns suggest that the w a d i deposits contain some M a s m a s formation material.


C o n s i d e r i n g the great similarities i n recent N i l e sediments, i t was not s u r p r i s i n g that the average concentrations of trace elements f r o m the various levels i n the K o m e l A h m r (Nekhen) area w e r e v e r y similar. A l t h o u g h U n i t s B , C , a n d N appear to be s i m i l a r to each other (Table III), especially i n the relative d i s t r i b u t i o n s of the R E E ( F i g u r e 5), there are some differences b e t w e e n these a n d the t y p i c a l N i l e silt ( N R S C ) for some elements. I f U n i t B is c o m p a r e d w i t h N R S C a n d the E l - K a b average, most of the differences can b e accounted for o n the basis of the d i l u t i o n b y sand. T h e d i l u t i o n effect of the coarser g r a i n e d m a t e r i a l (sand-sized fraction) can best be seen i n T a b l e I I I , as the m u d (made u p mostly of clay minerals) fraction (

Archaeological Chemistry IV - American Chemical Society

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